Mar 11, 2013

New research shows that cloud biases over the Southern Ocean are the primary contributor to the double-rain band problem that exists in most modern climate models.

It seems counterintuitive that clouds over the Southern Ocean, which circles Antarctica, would cause rain in Zambia or the tropical island of Java. But new research finds that one of the most persistent biases in global climate models – a phantom band of rainfall just south of the equator that does not occur in reality – is caused by poor simulation of the cloud cover thousands of miles farther to the south. University of Washington atmospheric scientists hope their results help explain why global climate models mistakenly duplicate the inter-tropical convergence zone, a band of heavy rainfall in the northern tropics, on the other side of the equator. The study appears this week in the Proceedings of the National Academy of Sciences.

"There have been tons of efforts to get the tropical precipitation right, but they have looked in the tropics only," said lead author Yen-Ting Hwang, a UW doctoral student in atmospheric sciences. She found the culprit in one of the most remote areas of the planet.

"What we found, and that was surprising to us, is the models tend to be not cloudy enough in the Southern Ocean so too much sunlight reaches the ocean surface and it gets too hot there," Hwang said. "People think of clouds locally, but we found that these changes spread into the lower latitudes."

Previous studies investigated tropical sea-surface temperatures, or ways to better represent tropical winds and clouds. But none managed to correctly simulate rainfall in the tropics – an important region for global climate predictions, since small shifts in rainfall patterns can have huge effects on climate and agriculture.

"The rain bands are very sharp in this area," commented co-author Dargan Frierson, a UW associate professor of atmospheric sciences. "You go from some of the rainiest places on Earth to some of the driest in just a few hundred kilometers."

"For the longest time we were expecting that it would be a combination of different factors," Frierson said, "but this one just stood out." The paper shows that cloud biases over the Southern Ocean are the primary contributor to the double-rain band problem that exists in most modern climate models.

"It almost correlates perfectly," Hwang said. "The models that are doing better in tropical rainfall are the ones that have more cloud cover in the Southern Ocean."

Hwang will speak on her results in April to scientists at the World Climate Research Programme. The results have also been submitted for inclusion in the fifth report of the Intergovernmental Panel on Climate Change, which is expected to appear next year.

Most models don't generate enough low-level clouds over the perpetually stormy Southern Ocean, the authors found, so heat accumulates in the Southern Hemisphere.

"Basically hot air rises, and it rains where air rises. So it's kind of obvious that the rain is going to be over warmer ocean temperatures," Frierson said. "Our new thinking is that the heat spreads – it's the warmth of the entire hemisphere that affects tropical rainfall."

In the short term, climate scientists can look for ways to improve the models to increase cloud cover over the Southern Ocean. Eventually, more powerful computers may permit models that are able to accurately simulate clouds over the entire planet.

"We have confidence in climate predictions outside the tropics, but tropical rainfall forecasts are much less certain," Frierson said. "We hope this work will lead to better rainfall forecasts in regions like equatorial Africa, where it's so important to have accurate predictions of future patterns."

Related Stories

Climate models project that the global average temperature will rise about 1°C by the middle of the century, if we continue with business as usual and emit greenhouse gases as we have been. The global average, ...

A new record of past temperature change in the tropical Atlantic Ocean's subsurface provides clues as to why the Earth's climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M ...

Recommended for you

An analysis of buildings tagged red and yellow by structural engineers after the August 2014 earthquake in Napa links pre-1950 buildings and the underlying sedimentary basin to the greatest shaking damage, ...

As everyone who lives in the San Francisco Bay Area knows, the Earth moves under our feet. But what about the stresses that cause earthquakes? How much is known about them? Until now, our understanding of ...

(Phys.org)—A trio of researchers with the Indian Institute of Science has found, via computer simulation, that deforestation in one part of the world can impact rainfall patterns in another. In their paper ...

It's no surprise that Arctic sea ice is thinning. What is new is just how long, how steadily, and how much it has declined. University of Washington researchers compiled modern and historic measurements to ...

Reasearchers at the University of Cadiz have carried out a study that establishes the atmospheric conditions responsible for the generation of extreme meteorological events in the Gulf of Cadiz, which can ...

Ok what happened to the guy who said "we have precise models that can mimic past changes in climate and are accurate enough to predict details regarding current and future climate?"

That "guy" does not exist, as anyone with a wit of intelligence would know. Models are only an approximation of the real world - an approximation that climate scientists are constantly trying to improve upon. Whoever led you to believe that forecasting the future is 100% correct? - that's just as impossible as being 100% wrong. Try looking at a normal distribution curve. If you understand one that is.

That guy may not exist, but the majority of people seem to think that climate models are a lot more accurate than they are. Those people include legislators and numerous planners and decision makers trying to figure out just how accurate climate change predictions might be and what actions need to be taken. In the last year their have been at least three major climate model glitches reported - not counting this one. We need to understand that models are not science in themselves, but are based on selected information (some times extrapolated and interpolated data) and employed potentially with bias. Much of the information used in climate models is outside of the accuracy of the instruments used. Examples being temps and sea level are predicting changes outside the range of accuracy of both satellite altimetry (fraction of mm.) and temp accuracy beyond the range of most recording temperature devices - i.e. 1.3 F is beyond the accuracy of the avg. field grade thermometer.

R2, you know very well that water vapor is taken into account for computer models of climate change! The funny little twist in the argument is that the climate models are actually UNDER predicting the global rise of temperatures from green house gasses. And wow, what an effect that will have on the denialist libertarian survivalist scourge. How do you deal with a film like "Beasts of the Southern Wild" that is so libertarian survivalist in its philosophy but so very concerned with the environment.

Shows the northern hemisphere bias in research. Nobody thought to do enough ground truth checking in the southern hemisphere (until now), because the southern weather patterns must surely be a mirror image of the northern hemisphere patterns. Mustn't they? Grrr... Not very happy with Yen-Ting Hwang's language "...so too much sunlight reaches the ocean surface and it gets too hot there," Hmmm. She better turn the southern hemisphere thermostat down then if it doesn't suit her.

Shows the northern hemisphere bias in research. Nobody thought to do enough ground truth checking in the southern hemisphere (until now), because the southern weather patterns must surely be a mirror image of the northern hemisphere patterns. Mustn't they? Grrr... Not very happy with Yen-Ting Hwang's language "...so too much sunlight reaches the ocean surface and it gets too hot there," Hmmm. She better turn the southern hemisphere thermostat down then if it doesn't suit her.

No, actually the SH circulation behaves very differently than that of the NH. Greater land-mass in the north with large mountain ranges disturbs/deflects the Jet-stream. SH is dominated by Antarctica at it's centre, with a vast expanse of ocean surrounding, keeping a tight zonal circulation.

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.

Javascript is currently disabled in your web browser. For full site functionality, it is necessary to enable Javascript.
In order to enable it, please see these instructions.